JP2012185020A - Nuclear fuel element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nuclear fuel element for a fast reactor using MOX pellets which extends a life of MOX fuel for the fast reactor and improves economical efficiency of a fast reactor cycle by properly arranging an oxygen getter member for absorbing excess oxygen generated during irradiation in a cladding tube.SOLUTION: A nuclear fuel element for a fast reactor has structure of arranging a core fuel 12 which comprises multiple MOX pellets in the axial center of a cladding tube 10, a lower axial blanket 14 under the core fuel, and an upper axial blanket 16 above the core fuel. A pellet-shaped oxygen getter member 26 which loads titanium or uranium metal as the oxygen getter member at low density is arranged above a MOX pellet 20 located at the uppermost stage of the core fuel via a heat insulation pellet 24, and the upper axial blanket is arranged on the oxygen getter member.

Description

  The present invention relates to a nuclear fuel element of a fast reactor using MOX pellets. More specifically, a low-density oxygen getter member using titanium or metal uranium is formed on a core fuel made of MOX pellets via a heat insulating pellet. The invention relates to a nuclear fuel element having a structure in which an upper axial blanket is disposed.

  In nuclear power generation, increasing the fuel life and reducing the frequency of fuel replacement are effective in improving the economy, and therefore, there is a strong demand for higher fuel burnup. In the case of the fast reactor, the current fuel life limiting factor is the swelling of the cladding tube, but improvement prospects are being obtained by the development of improved cladding materials such as oxide dispersion strengthened steel.

  On the other hand, in the fuel composed of uranium, plutonium oxide, and mixed oxides (Mixed Oxide: MOX), a stainless steel cladding tube is formed by the synergy of uranium and plutonium fission products and surplus oxygen generated during fission. Internal corrosion occurs. Since this inner surface corrosion progresses with the burnup degree of fuel, when the burnup degree becomes high, the soundness of the cladding tube cannot be maintained. As a result, when the swelling resistance is improved by adopting the improved coating material, it is expected that internal corrosion will prevent the extension of the burnup as a new life limiting factor, and the suppression thereof is an urgent issue.

Since uranium and plutonium oxides have non-stoichiometry, the composition of MOX fuel is expressed as MO ₂ ± _x . Here, 2 ± x which is the atomic ratio of oxygen and metal is called O / M ratio. The O / M ratio increases with the accumulation of surplus oxygen with the degree of combustion, and changes from 1.98 at the start of combustion to 1.99, 2.00, 2.01, for example. And from the vicinity of O / M ratio of 2.00, the internal corrosion of the stainless steel cladding tube becomes remarkable.

  As a measure for suppressing internal corrosion, a method of reducing the initial O / M ratio of the fuel to about 1.95, for example, is conceivable. In this case, the fuel O / M ratio is kept below 2.00 until a certain degree of burnup is reached, and the start time of internal corrosion can be delayed. However, it has been found that it is not always easy to manufacture the MOX fuel lowered to 1.95 compared to the O / M ratio of about 1.97.

  Therefore, a method has been considered in which an oxygen absorber is disposed in the nuclear fuel element, and the adsorbent absorbs excess oxygen to delay the increase in the O / M ratio. Such an oxygen absorbing material is called an oxygen getter, and several proposals have already been made. For example, there is a method of coating the inner surface of the cladding tube by composite Ni-based plating in which particles of getter material are dispersed (Patent Document 1), or a method of coating the surface of pellets with an oxygen getter material (Patent Document 2).

However, the former method of coating the inner surface of the cladding tube by composite Ni-based plating in which particles of getter material are dispersed has the following problems.
-From the center of the fuel element to the bottom, the cladding temperature is low, so the getter oxidation does not proceed, and at least titanium does not function as an oxygen absorber.
-The coating thickness needs to be increased accordingly, but the inner diameter of the cladding tube must be constant, so it is necessary to increase the overall thickness, and a wasteful getter is coated on the lower part.
・ Although the heat conduction of the metal coating is good, it is greatly reduced when it becomes an oxide, so the allowable thickness is limited.
・ Since the coating thickness is directly linked to the gap width of the pellet-cladding tube, a tight tolerance is set, but its precise control is difficult.

The latter method of coating the surface of the pellet with an oxygen getter material has the following problems.
-Since the coating thickness of the pellet is directly connected to the gap width of the pellet-cladding tube, strict control is still required, but this is difficult to realize.
・ Addition of coating process during mass production of pellets greatly impairs productivity.
・ Oxide fuel pellets with low thermal conductivity cause cracks due to thermal expansion at the start of the furnace and a large temperature gradient in the radial direction, and the getter material itself also undergoes volume expansion due to oxidation, which may cause peeling and dropping of the coating. There is.
・ The required amount of getter material cannot be loaded by coating only a part such as a blanket pellet.

  As another example, there is a method in which a porous thin pellet-shaped oxygen getter is disposed between blanket pellets disposed in the axial center of the nuclear fuel element (Patent Document 3). However, in the recent fast reactor core design, the core fuel is generally located at the axial center of the nuclear fuel element, and the blanket is arranged above and below the core fuel element. It is difficult to apply to elements. In addition, although there is a “porous thin pellet”, a specific manufacturing method is not disclosed at all, and it is difficult to manufacture metal pellets having extremely low density, and there is a problem in feasibility. In addition, the temperature of the cladding tube becomes higher as it goes to the upper part, and as a result, the inner surface corrosion becomes more severe as the upper part of the nuclear fuel element. Therefore, even if an oxygen getter is arranged in the central part in the axial direction, It is questioned whether it can be effectively suppressed.

  Thus, in any case, the above-described conventional technology is a pellet type fuel in which blanket pellets are arranged at both ends of a nuclear fuel element, which is a basic concept of a fast reactor MOX fuel that has been studied in Japan and overseas in recent years. When an oxygen getter is used, various conditions do not match and there is a problem in feasibility, and it is necessary to construct another concept.

JP-A-8-68885 JP-A-54-3696 JP 62-28688 A

  The problem to be solved by the present invention is that, in a nuclear fuel element in which blankets are arranged above and below a core fuel made of MOX pellets, an oxygen getter member for absorbing excess oxygen generated during irradiation is appropriately placed in an appropriate position in the cladding tube. By arranging in a state, it is intended to extend the life of the fast reactor MOX fuel and improve the economics of the fast reactor cycle.

  In the present invention, a core fuel composed of a large number of MOX pellets is located in the axial center of the cladding tube, a lower axial blanket is located below the core fuel, and an upper axial blanket is located above the core fuel. In the nuclear fuel elements for fast reactors, each of which is arranged, a pellet shape in which titanium or metal uranium is loaded at a low density as an oxygen getter material on top of the MOX pellet located at the top of the core fuel via an insulating pellet The nuclear fuel element is characterized in that an oxygen getter member is installed and an upper axial blanket is disposed on the oxygen getter member.

  Here, the oxygen getter member is preferably a titanium roll pellet in which a ribbon-like titanium foil is loosely wound with a gap. Instead of titanium roll pellets, wire bundle pellets in which titanium wires are loosely bundled may be used. As another form of the oxygen getter member, there is a structure in which metal uranium particles are accommodated in a cylindrical holder of a mesh-supported strut structure.

  Since the nuclear fuel element of the present invention has a structure in which an oxygen getter member is installed on an uppermost MOX pellet of the core fuel via a heat insulating pellet and an upper axial blanket is disposed on the oxygen getter member. In addition, the oxygen getter member can be installed in a region where internal corrosion is most prominent. In addition, by using a pellet-shaped oxygen getter member, it is possible to load a required amount of getter material even in a limited region, and by providing a heat insulating pellet, an optimum temperature is given to the oxygen getter member. And excess oxygen can be efficiently absorbed. Furthermore, by making the oxygen getter member into a pellet shape, it becomes possible to process the nuclear fuel element in a horizontally placed state, and the manufacturing process can be simplified.

  When the oxygen getter member is a titanium roll pellet in which a ribbon-like titanium foil is wound loosely with a gap, the smear density of the pellet can be arbitrarily adjusted by the thickness, length, and winding of the titanium foil, Low density pellets of 50% or less can be easily realized.

  When the oxygen getter member has a structure in which metal uranium particles are accommodated in a cylindrical holder with a mesh strut structure, the smear density can be arbitrarily adjusted by the amount of metal uranium particles accommodated, easily 50% or less Can be reduced in density.

Explanatory drawing of structure and temperature distribution of pellet type nuclear fuel element of fast reactor. Explanatory drawing of the nuclear fuel element which concerns on this invention. Explanatory drawing of the oxygen getter member which consists of a titanium roll pellet. The graph which shows the change of the titanium thickness by oxygen absorption. Explanatory drawing which shows the example of the oxygen getter member using a metal uranium particle.

First, the schematic structure and temperature distribution of the main part of a nuclear fuel element for a fast reactor using pellet type MOX fuel will be described with reference to FIG. In a recent fast reactor core design, as shown in FIG. 1A, the nuclear fuel element has a core fuel 12 positioned at the center in the axial direction of a long cladding tube 10, and a lower shaft below the core fuel 12. A directional blanket 14 and an upper axial blanket 16 are disposed above the core fuel 12. For example, the core fuel 12 portion is filled with a large number of MOX pellets 20 to a length of about 1000 mm, and the lower axial blanket 14 and the upper axial blanket 16 are made of UO ₂ pellets 22, both of which have the same length. About 200 mm. A gap having a specified dimension is formed between the outer peripheral surface of each pellet and the inner surface of the cladding tube.

  FIG. 1B shows an image of the temperature distribution of such a nuclear fuel element. The pellet center temperature exceeds 2000 ° C. during the transition in the MOX pellet near the center in the axial direction. The temperature of the cladding tube increases as it goes upward.

  The distribution image of the inner corrosion of the cladding and the getter temperature is shown in FIG. As described above, the temperature of the cladding tube increases as it goes upward, and as a result, (a) internal corrosion becomes more prominent at the upper part of the nuclear fuel element. (B) The temperature of the getter in contact with the pellet is excessive at the central portion in the axial direction and excessively low at both end portions, and there is a region of appropriate temperature between them. (C) The temperature of the getter at the gap has an appropriate temperature region in the vicinity of the upper part of the core fuel and the lower part of the upper axial blanket, but it is excessively lower above and below. This depends on whether the cladding tube temperature or the pellet temperature is governed.

  The present invention has been made in view of the inner corrosion of the cladding tube and the temperature distribution image in such a nuclear fuel element. As shown in FIG. 2, the nuclear fuel element for a fast reactor according to the present invention basically has a core fuel 12 formed by filling a large number of MOX pellets 20 in the axial center of the cladding tube 10, It is assumed that a lower axial blanket 14 is disposed below the core fuel 12 and an upper axial blanket 16 is disposed above the core fuel 12. Here, in the present invention, a pellet-shaped oxygen getter member 26 loaded with low density as titanium or metal uranium as an oxygen getter material on the MOX pellets positioned at the uppermost stage of the core fuel 12 via the heat insulating pellets 24. And the upper axial blanket 16 is disposed on the oxygen getter member 26. Both ends of the cladding tube 10 are sealed with a lower end plug 30 and an upper end plug 32, and a plenum spring 34 is loaded therein.

  Titanium or metal uranium is selected as the oxygen getter material in the present invention because it has a lower oxygen potential than MOX, has a small neutron absorption cross section, does not react with fuel and cladding, and expands due to oxidation and heat. This is because it is possible to satisfy the following conditions, such as not being excessive, not harmful when reprocessing spent fuel, and not causing melting or eutectic reaction at the temperature of the loading position.

  As described above, in the present invention, the pellet-shaped oxygen getter member 26 is installed on the core fuel 12 via the heat insulating pellets 24, and the upper axial blanket 16 is disposed thereon. The role of the oxygen getter is to protect the high temperature stainless steel cladding tube from the surplus oxygen and fission product compounds generated in the MOX, so the oxygen getter member should be loaded and placed as close as possible to the MOX pellet. Is desirable. On the other hand, since the central part of the core fuel exceeds 2000 ° C. during the transition, the oxygen getter is not allowed to contact. Within the nuclear fuel element, it is the upper axial blanket, as shown in FIG. 1B or C, where the getter temperature is neither too high nor too low. However, even in the region of the upper axial blanket, the temperature decreases in the upper direction, so that an appropriate temperature can be expected only in the lower direction. In order to load a necessary amount of getters in such a limited area, the getters have a pellet shape. The getter loaded in the region of the upper axial blanket has a problem that it is far from the MOX, but the internal corrosion is most noticeable in the upper axial blanket from above the core fuel where the cladding tube becomes the highest temperature (FIG. 1). C)), the getter placed in the immediate vicinity of the position functions effectively. By the way, although a moderate temperature can be expected even in the region of the lower axial blanket, the lower axial blanket is obviously not far from the corroded region and thus is not suitable as a loading place for the getter.

Here, the temperature of the core fuel is still high even in the uppermost MOX pellet, and when the getter material comes into contact, melting and reaction occur, and therefore a heat insulating pellet (UO ₂ pellet) 24 is sandwiched between them. If the two are separated too much, the temperature of the getter material becomes low, making it difficult to efficiently absorb excess oxygen. Therefore, an optimal temperature is applied to the oxygen getter member by appropriately selecting the thickness and number of the heat insulating pellets.

  With respect to the oxygen getter member 26, it is necessary to cope with volume expansion. Titanium or uranium expands to about twice the volume before oxidation when it becomes a dioxide. Therefore, in order to avoid excessive stress with the cladding tube, the loading density (smear density) is about 50% or less (for example, 50%). It must be loaded at a low density (about 40%).

  Therefore, in the present invention, a titanium roll pellet obtained by sparsely winding a ribbon-like titanium foil with a gap is used as the oxygen getter member. Thus, a low-density pellet can be obtained by loosely winding a foil-like getter material into a roll.

  Instead of such titanium roll pellets, a structure in which titanium wire is bundled is also effective. The wire is not linear but corrugated, or by having irregularities in the cross-sectional shape, so that it does not become a dense bundle and can be a low-density pellet.

  The oxygen getter member may have a structure in which metal uranium particles are accommodated in a cylindrical holder having a mesh strut structure. By using a strut-structured holder, it has mechanical strength that can withstand the spring force of the plenum spring, and by using a mesh tension structure, the plenum gas permeability is secured and the volume expansion of metal uranium particles is covered with the holder. Can escape into the gap between the tubes.

  An example of a pellet-shaped oxygen getter member used in the present invention is shown in FIG. This is an example of the titanium roll pellet 40, and as shown in A, it is a structure in which a ribbon-like titanium foil 42 is wound loosely and loosely with a gap. The nuclear fuel element is similar to that shown in FIG. 2 and is loaded with titanium roll pellets as its oxygen getter member.

  Titanium roll pellets with an outer diameter of 8.8 mm (maximum value) were prototyped on the assumption that they were loaded onto a cladding tube with an inner diameter of 8.98 mm, which was a practical fuel specification. In order to obtain a smear density of 50% with respect to the outer diameter of 8.8 mm, a titanium foil having a thickness of 100 μm was cut into a width of 10 mm and a length of 284 mm, and wound around a jig having an outer diameter of 3 mm to form a roll. The obtained outer diameter was 9.39 mm, and this was stored in a jig having an inner diameter of 8.8 mmφ and subjected to heat treatment at 800 ° C. to obtain a roll having an outer diameter of 8.64 mm. FIG. 3B shows a state in which titanium roll pellets are loaded on the cladding tube. As shown in an enlarged view in FIG. 3C, a gap was formed between the titanium foils, and air permeability was secured.

The MOX amount per nuclear fuel element (cladding tube diameter 10.4 mmφ) of a practical fast reactor is assumed to be 2.11 mol. In order to absorb the surplus oxygen corresponding to the O / M ratio of 0.08 toward the target burnup, it is necessary to load 0.0844 moles of oxygen getter material whose oxidation form is dioxide, per fuel element, In the case of titanium, 0.9 cm ³ may be loaded. Since the true density of a titanium roll pellet having an outer dimension of 8.8 mmφ × 10 mmH and a smear density of 50% is 0.317 cm ³ , it can be seen that it is only necessary to load three titanium roll pellets per nuclear fuel element.

  FIG. 4 shows the measurement results of the oxygen absorption state of titanium. When a titanium disk (10 mmφ × 3 mmt) is heated at 700 ° C. for up to 200 hours at −347 kJ / mol, which is a typical oxygen potential of MOX, the result is that the thickness of the titanium oxide layer reaches a maximum of 209 μm. It was. From this, when oxidation proceeds from both sides of a 100 μm thick titanium foil, it is expected that the whole will oxidize in about 50 hours, and can absorb surplus oxygen at a sufficiently high rate compared to the cladding corrosion that proceeds monthly. Was confirmed.

Moreover, as a result of evaluating the reactivity of titanium and UO ₂ at 1300 ° C., it was confirmed that the coexistence of both was obtained at around 700 ° C. Furthermore, as a result of evaluating the reactivity between titanium and stainless steel (ferrite-martensitic steel) at 700 ° C., it was also confirmed that no reaction affecting the soundness of stainless steel occurred.

Such titanium roll pellets are
・ Titanium foil with a thickness of several tens to several hundreds of μm is available, and it is easy to cut it into a ribbon and wind it into a roll.
・ The smear density of the pellets can be arbitrarily adjusted by adjusting the thickness, length, and winding of the titanium foil.
-Since the volume expansion when titanium is oxidized can be absorbed in the gap between the foils, the interaction with the cladding tube can be avoided even when the temperature is low,
-The oxygen absorption efficiency is high due to the large specific surface area.
・ Excellent handling and can be loaded with cladding tube in the horizontal position
It has been confirmed that it is useful as an oxygen getter member.

  From these results, the nuclear fuel element of the present invention using titanium roll pellets as an oxygen getter member can extend the life of the fast reactor MOX fuel and improve the economics of the fast reactor cycle.

  Another example of the pellet-shaped oxygen getter member used in the present invention is shown in FIG. This is an example in which metal uranium particles are used. As shown in FIG. 5A, the metal uranium particles 52 are accommodated in a cylindrical holder 50 having a mesh strut structure. The nuclear fuel element is the same as that shown in FIG. 2, and an oxygen getter member containing metal uranium particles 52 in a holder 50 is used.

  The holder 50 has a structure in which a circular end plate 54 is positioned at an interval in the vertical direction and is supported by a plurality of support posts 56 therebetween. A plurality of (three in FIG. 5) support columns 56 are provided symmetrically along the outer peripheral edges of both end plates 54 and are surrounded by a mesh to hold the internal metal uranium particles 52. As the holder material, for example, titanium is preferable. This is because steel has a problem of causing a eutectic reaction between uranium and iron.

When using metal particles as the getter material as in this embodiment, there are the following problems.
-Since metal particles are supplied as a product with a density of 100%, a filling rate of 50% is necessary to obtain a smear density of 50%. However, since the packing density of the spherical particles in the narrow tube reaches a maximum of 62.2%, under the situation where each member in the cladding tube is restrained by the plenum spring, the packing density is as low as 50% by directly filling the getter particles. Can't get.
・ Processing of nuclear fuel elements is carried out horizontally for the convenience of filling pellets, etc., but in order to fill the getter particles, it must be placed vertically, and then again to fill the shaft bra pellets. Since it must be placed horizontally, the manufacturing process becomes complicated.

When the metal uranium particles are accommodated in a holder and loaded on the cladding tube together with the holder as in the present invention, all of these problems can be solved. By using a mesh strut holder, the mechanical strength to withstand the spring force of the plenum spring is provided, and the air permeability of the plenum gas and the volume expansion of the metal uranium particles are increased between the holder and the cladding tube. (See B in FIG. 5). In addition, about the metal uranium particle | grains, there exists a track record in which the product under 100 micrometers was manufactured in Japan, and there is no problem in particular in manufacture of an oxygen getter member. In the present invention, since the temperature of the blanket is lower than that of the core fuel, and the contact between the MOX and UO ₂ and the getter is hindered by the holder, the eutectic reaction can be avoided.

  Oxygen getter members such as titanium roll pellets, titanium wire bundle pellets and metal uranium particles used in the present invention are not limited to pellet type fuels but also to granular fuels such as vibration-filled fuels. Applicable.

DESCRIPTION OF SYMBOLS 10 Cladding tube 12 Core fuel 14 Lower axial blanket 16 Upper axial blanket 20 MOX pellet 22 UO ₂ pellet 24 Heat insulation pellet 26 Oxygen getter member

Claims (3)

A core fuel composed of a large number of MOX pellets is located in the axial center of the cladding tube, a lower axial blanket is disposed below the core fuel, and an upper axial blanket is disposed above the core fuel. In the nuclear fuel element for the fast reactor of the structure,
On the MOX pellet located at the uppermost stage of the core fuel, a pellet-shaped oxygen getter member loaded with low density using titanium or metal uranium as an oxygen getter material is installed via a heat insulating pellet. A nuclear fuel element comprising an upper axial blanket disposed thereon.   2. The nuclear fuel element according to claim 1, wherein the oxygen getter member is a titanium roll pellet in which a ribbon-like titanium foil is loosely wound with a gap.   The nuclear fuel element according to claim 1, wherein the oxygen getter member has a structure in which metal uranium particles are accommodated in a cylindrical holder having a mesh strut structure.

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